Generally, in order to form a wiring pattern on a surface of a semiconductor wafer serving as an object to be processed or bury recesses between wiring or contact holes during a semiconductor integrated circuit manufacturing process, a metal or a metalic compound such as W(tungsten), WSi(tungsten silicide), Ti(titanium), TiN(titanium nitride), TiSi(titanium silicide), Cu(copper) or Ta2O5(tantalum oxide) is deposited to form a thin film. In such a case, a tungsten film is widely uses since it has a small resistivity and requires a low film adhesion temperature. In order to form such a tungsten film, tungsten hexafluoride (WF6) is used as a source gas and is reduced by hydrogen, silane, dichlorosilane or the like, thereby depositing the tungsten film.
In case of forming such a tungsten film, a Ti film, a TiN film, or a stack of both of those films is thinly and uniformly formed first on a wafer surface as a barrier layer serving as an under film for the purpose of improving the adhesivity and then suppressing a reaction with an underlying silicon layer and the tungsten film is deposited thereon.
When burying or remedying the recesses or the like, hydrogen gas having a weaker reducing power than silane is mainly used to enhance the burying characteristics. At this time, the barrier layer may be attacked by unreacted WFs to react with fluorine and then be expanded in volume, thereby generating an upwardly protruded volcano or a void in a buried hole.
The above phenomenon will be explained with reference to FIG. 13. FIG. 13 is a cross sectional view of a buried hole having a volcano and a void. A buried hole 2 such as a contact hole or the like is formed on a surface of a semiconductor wafer W. A barrier layer 4 formed of, e.g., a Ti/TiN film, is formed in advance on the surface including an inner surface of the buried hole 2. In case of performing a burying process on the structure described above by depositing a tungsten film 6 by way of simultaneously supplying WF6 gas and H2 gas, fluorine of the WF6 gas is diffused into the barrier layer 4. Especially, the fluorine reacts with Ti in the barrier layer 4 on the inner surface, leading to a protruded deposition of the tungsten film 6 starting at the buried hole 2. As a result, a volcano 8 can be generated at an end portion of the protrusion by a stress of the tungsten film 6 or a void 10 having a cavity shape can also be generated inside the buried hole 2.
In order to prevent the generation of the volcano 8 or the like, silane having a stronger reducing power than that of the hydrogen gas was also used for forming a nucleation layer of the tungsten film 6 with a small thickness of, e.g., about 300 to 500 Å. Thereafter, deposition of a main tungsten film was carried out starting at the nucleation layer by using H2 gas and WF6 gas. In this case, however, the nucleation layer may not be Uniformly formed due to, e.g., a contamination of a surface of the barrier layer 4 serving as a base film.
Thus, prior to the formation of the nucleation layer, only silane is provided for a certain time period to allow reaction intermediates of the silane (SiHx:x<4) to be absorbed on a wafer surface at a low temperature, e.g., 400° C. at which a part of the silane can be decomposed. Then, the nucleation layer is grown starting at that portion. FIGS. 14A to 14F illustrate the processes for charging a buried hole with tungsten by using the above-described method.
As shown in FIG. 14A, an initiation process of adhering reaction intermediates 12, i.e., SiHx discussed above, on a surface of a wafer W is performed on the wafer W having a barrier layer 4 formed on its entire surface including an inner surface of a buried hole 2 (FIGS. 14A and 14B). Next, as described above, by simultaneously supplying the WF6 gas and the SiH4 gas for a certain time period as illustrated in FIG. 14C, a tungsten film is deposited starting at the reaction intermediates 12, thereby forming a nucleation layer 14 (FIG. 14D).
Subsequently, by simultaneously supplying the WF6 gas and the H2 gas as shown in FIG. 14E, a main tungsten film 16 is deposited so as to fill the buried hole as illustrated in FIG. 14F.
In case of forming the barrier layer 4 formed on the wafer surface, an organic compound source of Ti is normally used in order to increase a step coverage. However, a carbon component in the organic compound source is included in the barrier layer 4 and, thus, adhesion of the reaction intermediates to the barrier layer 4 becomes irregular despite the initiation process due to the exposure of the carbon component on a surface of the barrier layer 4. Consequently, the nucleation layer 14 is irregularly formed thereon, and the step coverage thereof is also deteriorated, resulting in deteriorated burying characteristics of a main tungsten film, causing voids, volcanoes or the like.
Such a problem does not occur in a case where the ratio between the thickness of the nucleation layer 14 and that of the entire tungsten film including the main tungsten layer 16 is not so high. On the other hand, in case the ration becomes non-negligibly high because of scaling down, voids of a non-negligible size can be generated due to a deterioration of the step coverage of the nucleation layer 14.
The above problem becomes aggravated as a serious issue especially when an inner diameter of a buried hole becomes smaller than or same to, e.g., 0.2 μm as a result of the manufacturing requirement for a further scaling down and a thinner film.